Measurement of CO2 concentration in pipeline streams is useful and necessary for a variety of reasons. The heating value of the natural gas stream, or more appropriately the standard heat of combustion, deteriorates with increasing CO2 concentration.
In many CO2 injection projects, methane is present in a recycle stream. Composition of the gas stream is regularly monitored for both separator operation and termination of inefficient recycling of CO2. Furthermore, in some applications, the phase behavior of the gas stream is altered sufficiently to have a material effect on operational design.
In contrast, many gas wells produce CO2 with methane and other gaseous light hydrocarbons. In these applications, a downhole measurement of periodic gas samples or an in-line continuous measurement is valuable. Likewise, in monitoring applications for fields undergoing CO2 injection, a robust method capable of quantitative determination for evaluating migration of CO2 is essential. In these applications, sensor sensitivity spanning the entire range of mole fractions is required.
The Japanese patent application publication number 2000275213A uses a dual molten carbonate cell where a first carbonate is dissociated to generate CO2 and O2 by applying a small current of about 10 mA. Another cell is used to measure the junction potential across the second cell's electrolyte in contact with the reference gas stream and the sampled gas. The interpretation of the measured potential relies on small levels of CO2 concentration, typical of what is found in the atmosphere. The mechanical assembly is not designed to withstand a pressure differential between the reference and sampling compartments.
The Japanese patent application publication number 2004170230A proposes a CO2 sensor using electrodes with a film coated with a thick conductive ceramic, NASICON™ This type of apparatus generally suffers from limited performance in the presence of water vapor due to the deterioration of NASICON™ with moisture.
United States Patent Application Publication Number 20090095626A1 uses a sensor structure containing lithium phosphate as electrolyte and a mixture of lithium and barium carbonates as electrode surface coating materials. The reference electrode is a lithium titanate-titanium dioxide mixture. It operates at 500° C. This application's focus is structuring a carbonate electrode layer that had reduced sensitivity to humidity.
These references are applicable when the systems are at a low pressure, and the systems are designed to operate for dilute CO2 concentration in the gas stream. Furthermore, the sensors are slow to respond, often requiring minutes. Given the corrosive nature of the electrolyte, feeding two different gas streams and preventing electrolyte migration beyond its housing remains an issue.